The present invention relates to a plasma processing apparatus and plasma processing method for processing a sample in the form of a substrate such as a semiconductor wafer placed in a processing chamber formed within a vacuum vessel by the use of a plasma generated in the processing chamber and, more particularly, to an inductively coupled plasma processing apparatus and processing method capable of exciting a plasma at a quite low potential adapted for processing such as etching, ashing, CVD (chemical vapor deposition) using a helical coil resonating in a full wavelength mode.
In processes for mass producing semiconductor devices, plasma processing such as plasma etching, plasma CVD, and plasma ashing has enjoyed wide acceptance. Plasma processing is carried out by generating a plasma by applying RF electric power or microwave electric power to a process gas under an evacuated state and directing a stream of ions or radicals at a wafer. Semiconductor devices will continue to be fabricated with still reduced dimensions. According to the International Technology Roadmap for Semiconductors (ITRS), it is expected that devices having half pitches equal to or less than 20 nm will begin to be mass produced between 2014 and 2016. It is expected that the main transistor structure adopted at this time will the Fin FET type of 3D structure such as double gate type and trigate type rather than the planar type (flat surface type) of the current mainstream. Plasma processing apparatuses and equipment used for manufacturing of these future semiconductor devices are required to have finer processing performance, lower damage, selectivity, controllability, and stability. Especially, as device dimensions decrease further, damage caused by a plasma during the processing will become more serious.
There are some types of damage caused by plasmas. It is known that as the plasma potential rises, various harmful effects are produced. For example, in an ashing process carried out by introducing a gas such as O2 from the top of a plasma reactor and causing O radicals generated by a plasma to react with a resist lying on a wafer placed in a downstream region, the wafer is normally mounted on a wafer stage at ground potential. Therefore, where the plasma potential is high, a potential difference is created relative to the wafer. Positive ions within the plasma are accelerated and enter the wafer. As a result, the underlying film may be damaged or peeled off. Furthermore, depending on high plasma potentials, a plasma may be generated in inherently undesired locations, e.g., beside or below the stage. Accordingly, the plasma potential needs to be controlled to low values substantially close to the ground potential.
One example of the prior art capable of achieving low-damage processing by suppressing the plasma potential is a technique disclosed in JP-A-2000-501568 (corresponding to U.S. Pat. No. 5,965,034), where a helical resonator is applied to plasma generation. In the present technique, electrical current and voltage standing waves are excited by helical coils resonating in a full wavelength mode. The phase voltage of the voltage standing waves and the reverse-phase voltage are made to cancel each other. At the point at which the phase voltage is switched, i.e., at a node where the potential is substantially zero, a plasma at a quite low potential can be excited by induction coupling owing to electrical current standing wave. Consequently, capacitive coupling between a voltage induced in a coil and a plasma can be prevented; in the conventional inductively coupled plasma source, it would have been difficult to avoid such capacitive coupling. Hence, increases in the plasma potential can be suppressed.
Furthermore, JP-A-2000-49000 discloses a technique in which a frequency matching device is employed in a plasma processing apparatus using the aforementioned helical resonator. In this technique, electric power reflected from a plasma load or the phase difference between voltage and current is monitored, and reflection from the load is automatically suppressed by providing feedback control of the oscillation frequency from a power supply such that the reflected electric power is reduced or the phase difference between the voltage and current is reduced down to zero.
In addition, JP-A-2003-37101 discloses a technique in which a load impedance is made to correspond to some different types of processing conditions by switching the ground point of a resonance coil by the use of a vacuum relay in a plasma processing apparatus that uses the above-described helical resonator and frequency matching device.